1. Technical Field of the Invention
The present invention relates to a content addressable memory (CAM) and, in particular, to a device for battery back-up protection of CAM stored data.
2. Description of Related Art
Content addressable memories (CAMs) are well known in the art. A CAM is a memory device in which data is stored in word locations comprising multiple memory cells, each cell storing a single bit of information. A search word in a comparand register of a CAM may be simultaneously compared with all stored data words. If a comparison for only a portion of a stored data word is desired, one or more mask registers may be used to prevent particular bits within the data word or words from taking part in the comparison with the search word. When a match occurs between the search word and one or more of the CAM stored data words, a match signal is generated. The address of the matching location within the CAM is then determined from the match signal and output. Thus, unlike the data accessing operation performed in a random access memory (RAM) where a user supplies an address to obtain stored data, a CAM-based device operates by having the user supply the data to obtain an address.
Not with standing this operational distinction between RAM-and CAM-based devices, these memory devices do share at least one important common operational featurexe2x80x94data volatility. If there is a power failure with respect to the applied voltage (Vcc) for either a RAM- or CAM-based device, the data stored by the device is irretrievably lost. In many system implementations where CAMs are utilized, this potential data loss presents a significant concern. For example, CAM-based devices are utilized in many communications switches/routers to store address translation tables. In the event of a power loss for the switch/router system that corrupts or destroys the stored data, the address tables must be re-built or recovered. This can be quite an involved and time consuming task.
There is accordingly a need for a circuit that functions to protect CAM-based device stored data in the event of a power disruption.
An external voltage is compared to an enable reference voltage. If the external voltage drops below the enable reference voltage, a content addressable memory (CAM) is placed into a low current, stand-by mode of operation. The external voltage is further compared to a supply reference voltage. If the external voltage drops below the supply reference voltage, the voltage power supply for the CAM is switched from external voltage input to a battery back-up.
In an alternative embodiment, the CAM includes a separate power input for a CAM array, and the switching action causes only that separate power input for the CAM array to be powered from the battery back-up.